In 1966, when the Arno flooded in Florence, Vasari’s “The Last Supper,” hanging in the museum of the Church of Santa Croce only a few blocks from the river, was among the many masterworks that was ruined. This panoramic scene, almost 21 feet long and 8 feet tall, remained submerged in muddy, oily water for a dozen hours, and when it was finally rescued, its poplar panels were as damp as sponges, the paint atop them peeling off. It’s now being restored, in hopes to show it to the public in 2016, for the 50th anniversary of the flood.